During the past year, we were able to establish the first viable mouse model of XNDI in which the V2R gene can be deleted in a conditional (4-OH-tamoxifen-inducible) fashion in the kidneys of adult mice. The resulting V2R KO mice showed all key symptoms of XNDI, including the production of large amounts of dilute urine (polyuria) and polydipsia. Kidneys from V2R KO mice also displayed a distension of the renal pelvis, a characteristic morphological deficit that is typically associated with XNDI. Moreover, the renal expression levels of AQP2 and AQP3, two water channels that play a key role in mediating V2R-dependent water reabsorption, were dramatically reduced in the V2R KO mice. Gene expression analysis showed that mouse inner medullary collecting cells express another Gs-coupled receptor (besides the V2R), the EP4 prostanoid receptor, at significant levels. To test the potential usefulness of EP4 receptor agonists for the therapy of XNDI, we treated V2R KO mice with the selective EP-4 receptor agonist, ONO-AE1-329 (ONO). Remarkably, both acute and chronic treatment of V2R mutant mice with ONO greatly reduced all major manifestations of XNDI, leading to striking reductions in urine output and water intake and pronounced increases in urine osmolality. Moreover, MRI studies showed that prolonged ONO treatment of V2R KO mice could prevent the further progression of renal pelvic distension normally seen with V2R KO mice. Treatment of V2R KO mice with ONO also led a significant increase in renal AQP2 levels. We also demonstrated that ONO treatment of kidney collecting duct tubule preparations led to a pronounced increase in cAMP levels and enhanced water permeability. Taken together, these findings strongly suggest that the beneficial effects of ONO in the treatment of XNDI mice depend on its ability to activate EP4 receptors expressed by renal collecting duct cells, raising the possibility that selective EP4 receptor agonists may prove useful for the treatment of human XNDI. We are currently using these newly generated XNDI mutant mice to screen for drugs that may have potential in the treatment of human XNDI. Such drugs include agents that have been reported to be endowed with antidiuretic properties or to have beneficial effects on AQP2 expression and trafficking. Moreover, after having analyzed the GPCR expression pattern in mouse collecting duct tissue, we are also testing drugs acting on other Gs-coupled receptors that may ameliorate XNDI symptoms by activation of the Gs-PKA signaling cascade.